Using a blender or food processor, puree 1-2 cups of fresh blueberries. Sieve the mixture to remove seeds and then cook it in a pot over medium heat, until it’s reduced, thickened and much of the water has evaporated. You’ll need ½ cup for the cake. Let it cool fully before adding to the cake batter.
Preheat the oven to 325 F. Butter and flour two 8” round cake pans.
In the bowl of a stand mixer fitted with the paddle attachment, beat together the butter, sugar, cake flour, baking powder, soda and salt until the mixture is fully combined and sandy.
Add the eggs, one at a time, allowing for each to be fully combined before adding another. Once all four are in, let the mixture run for a few minutes, scraping the bottom of the bowl as needed.
With the mixer on, slowly pour in the blueberry reduction, blueberry powder, lemon juice and kefir. Once the batter is fully mixed, divide it between the two cake pans.
Bake until top is firm when pressed or a cake tester comes out clean, about 40 minutes.